This application relates to a planet gear for incorporation into an air turbine starter.
Planetary gear systems are well known, and have been utilized to provide a speed change between an input and an output. A sun gear rotates about a central axis, and a ring gear rotates outwardly of the sun gear. A plurality of planet gears are positioned to transmit rotation between the sun and ring gears.
In one known type of planetary gear systems, the planet gears are mounted on stationary shafts positioned inwardly of the gears. The planet shaft provides an inner ring for bearings which support the planet gears.
One application of a planetary gear system is in an air turbine starter system. In an air turbine starter system, air is delivered across a turbine rotor to drive the rotor. The rotor drives a sun gear, which drives a ring gear through planet gears. The ring gear in turn drives a starter shaft for a gas turbine engine.
In one known air turbine starter system, the planet gears are mounted on a stationary shaft, and there are needle bearings positioned between the shaft and the interior of the planet gear. It is known to provide a surface treatment to the inner periphery of the planet gear to harden the surface such that the needle bearings are able to successfully run relative to the surface. One known hardening surface provides a carburized surface on the inner diameter of the gear.
However, in at least one known planet gear, the input teeth and the output teeth are provided by two separate gear portions which are secured together. In this known prior art, the two parts have been welded. The weld joint occurs at the outer periphery of a gear shaft, and relatively radially close to the inner periphery which is carburized. There has sometimes been a loss of hardness at the inner peripheral surface.